As altitude increases, atmospheric pressure decreases. Low pressure areas (i.e. at high altitudes) have less atmospheric mass, whereas higher pressure areas have greater atmospheric mass. Therefore, most modern aircraft and in particular, commercial passenger aircraft have pressurized cabins that reduce the effective altitude experienced within the aircraft, while flying at higher altitudes. When an aircraft's cabin and flight deck's effective altitudes are reduced, the total pressure of the interior of the aircraft is increased. This leads to a higher differential pressure between the inside and outside of the aircraft, with the stress becoming greater as the differential pressure increases. In order to reduce the effective altitude within the airplane, either the structure of the aircraft would need to be redesigned or adjusted to safely withstand the higher pressure, or the aircraft is flown at a lower altitude. Also, aircraft flown at higher differential pressures require increased maintenance and inspection, which will result in increased cost.
The effective altitude within the aircraft experienced by users such as passengers, at selected locations on the aircraft, can be reduced, without increasing the total pressure, by increasing the oxygen partial pressure in those locations, to an equivalent lower altitude value. Low oxygen and humidity levels which may be encountered during flight at increased effective cabin altitudes in an aircraft, may contribute to various adverse health effects, including light-headedness, loss of appetite, shallow breathing and difficulty in concentrating. For example, ascent from ground level to 8000 ft. pressure altitude lowers oxygen saturation in the blood by ˜4% (e.g. Muhm 2007). Dehydration is another adverse health effect, due to the dryness of the air. A human's preferred level is approximately 40-60% relative humidity, and in-flight humidity can drop below 10%. A dry thin atmosphere can also cause disturbed sleep patterns and can result in lack of energy, headaches, nausea, and loss of appetite.
Many commercial and other aircraft are equipped with gas separation systems such as nitrogen generating systems (NGS) to generate nitrogen enriched air that is channeled into parts of the aircraft, such as fuel tanks, for creating an inert atmosphere. The nitrogen generating system also produces oxygen enriched air. However, the oxygen enriched air from the nitrogen generating system is not used, typically being released overboard. The nitrogen generating system can receive bleed air flowing from at least one engine of the aircraft, or from a compressor or other source on board the aircraft. During all phases of flight, a portion of the air flow used in the nitrogen generating system is discarded in the form of oxygen enriched air. The air that is released overboard without being used causes an unnecessary drain on the aircraft systems reducing efficiency.